wave-xmpp@appspot.com is a robot which allows you to subscribe to waves so that your receive notifications via XMPP when users modify them.
You need to configure your XMPP client so that ...@wavesandbox.com can receive IM messages.
Trillian, Google Talk and Pidgin are confirmed to work with this.
Upon your first subscription you will receive an invitation, else you just have to add wave-xmpp@appspot.com to your contact list and not block it.
To use it, invite wave-xmpp@appspot.com as a participant to a wave. It will then insert a menu with two buttons. One for subscribing, one for unsubscribing.
If there are no buttons in the wave, because they have been deleted, type x:menu [xmpp_m] into an empty blip.
You can also type x:sub [xmpp_s] or x:unsub [xmpp_u] into an empty blip to subscribe or unsubscribe, respectively.